Processes for producing C/SiC components have been known, for example from the patent application DE-A 197 10 105, and comprise, inter alia, the following steps:                production of a mouldable mixture of, firstly, carbon-containing fibers or fiber bundles (hereinafter referred to collectively as “fiber material”), where the fibers may be provided with a coating, and, secondly, fillers and/or binders such as resins and/or pitch,        shaping of the mouldable mixture under pressure and at elevated temperature, and carbonization of the carbon-containing fillers and binders to produce a shaped body, in particular a shaped body consisting of carbon reinforced with carbon fibers (C/C), and, if desired, graphitization of the shaped body,        infiltration of at least an outer layer of the shaped body with a silicon melt and at least partial reaction of the silicon with the carbon in the shaped body to form SiC, thus producing a shaped body which consists, at least in the outer layer, of a composite ceramic having carbon-containing fibers embedded in a matrix comprising predominantly SiC, Si and C (here also referred to as C/SiC).        
In the following, “C/SiC” should in general be taken to encompass materials variants in which, as described above, only an outer layer is silicized.
Present-day metallic brake disks frequently have ventilation slots or channels within the disk through which air flows in order to reduce the temperature level of the disk and decrease the wear of the friction linings under high stress. Such ventilation channels are also provided in brake disks based on C/SiC, especially to reduce the temperature level so as to spare the brake linings and further system components.
A process for producing friction units comprising C/C—SiC material with ventilation channels, hollow spaces and recesses, in which a structured porous carbon body close to its final shape is infiltrated with liquid silicon, has been known from EP-B 0 788 468. This process makes use of the fact that infiltration with liquid silicon and formation of the Si- and SiC-rich matrix of the composite proceeds to completion virtually without any change in the geometry of the C/C intermediate body, so that the hollow spaces and recesses can be produced in the relatively soft and readily machinable C/C intermediate body and not only in the very hard C/C—SiC composite ceramic. It is proposed, inter alia, that the hollow spaces and recesses be formed by means of soluble cores of polystyrene foam or other rigid foams, by means of pyrolysable cores of polyvinyl alcohol or by means of rubber, metal or ceramic cores which can be taken out. The material of the cores forms the ventilation channels of the friction unit, with the webs between the individual ventilation channels being defined by the appropriate voids within the core material.
In the German patent application DE A 102 34 400, a process for producing hollow bodies is disclosed in which cores of compressible materials which have been compressed by at least 5% of their length in the pressing direction during pressing are used. The cores can be made up of a plurality of layers, with the outer (covering) layers being stiff and the inner layer being compressible.
Although this process allows better compaction of the mouldable mixture in the region of the webs of the pressed body which are formed in the position of the voids in the cores, it has been found that both in the case of the single-layer embodiment and in the case of the multilayer embodiment described having a compressible layer in the interior and stiff covering layers, compression of the core material exerts a pressure on the sides of the webs which, particularly in the case of narrow webs and wide voids, can lead to undesirable deformation.
It is therefore an object of the invention to provide a process and develop a core material matched thereto, by means of which it is possible to obtain fiber-reinforced hollow ceramic bodies which have uniform densification over the entire body and by means of which even narrow webs with pre-defined contours can be produced in the interior without deformation. Furthermore, the cores should be such that they can be removed gently and in a simple manner from the hollow body.